1. Field of Invention
The invention relates to imaging systems and methods. More specifically, the invention relates to imaging systems and methods that discharge an electrical charge stored in an image sensor.
2. Description of Related Art
Selenium-based X-ray imaging devices are commonly used in imaging systems to directly convert X-rays into an electrical charge. The electrical charge may be read out using conventional TFT switches and charge-sensitive amplifiers. Selenium has a high resistance value which provides a benefit in using selenium based X-ray imaging devices. Moreover, because the electrical charge does not spread laterally, an image with a high resolution may be obtained. These benefits make selenium-based imaging devices ideal, for example, in mammogram applications where the resolution of approximately 50 microns is desirable.
However, a disadvantage in using selenium is its propensity to trap or retain an electrical charge for a longer duration that desired, and in some instances, it requires many seconds to remove the electrical charge. The trapped electrical charges may disturb subsequent cycles of image formation. Thus, the trapped electrical charges need to be removed quickly. The trapped electrical charges are exacerbated in configurations where insulating layers are located between the selenium and its electrical contacts. The insulating layers are used to block electrical charge injection from the electrical contacts and to reduce dark current. However, this configuration also tends to prevent any electrical charge flow into the selenium that may neutralize the trapped electrical charges in the sensors.
One conventional device that removes the trapped electrical charges uses a back-illumination of a selenium array. This device uses a transparent glass substrate underneath the selenium array. However, the device requires a transparent pixel structure so that the back-illuminated light may pass through the pixel structure to the selenium. The problem with this arrangement is that the pixel structure needs to include a very large storage capacitor (nearly the entire length area of the pixel) to remove the electrical signals trapped in the selenium. The storage capacitors are conventionally formed between gate and data metal electrodes using the gate insulating layer as the capacitor dielectric. If the pixel is transparent, then both the bottom and top metals of the storage capacitor need to be formed of transparent material, for example, indium tin oxide (ITO). This configuration requires additional steps for forming the storage capacitors and results in a more costly device.